Flexible bioelectronic photodetector and imaging arrays based on bacteriorhodopsin (BR) thin films

ABSTRACT

The direct deposit of photoelectric materials onto low-cost prefabricated patterned flexible electrodes provided by the present invention introduces a new design approach that permits the development of innovative lightweight, durable and non-planar sensing systems. By extending single and multi-spectral bioelectronic sensing technology to flexible plastic substrates, the invention offers a number of potential advantages over structurally rigid silicon-based microelectronics (e.g. CMOS) including a reduction in spatial requirements, weight, electrical power consumption, heat loss, system complexity, and fabrication cost.

CROSS REFERENCE TO RELATED U.S. APPLICATION

This patent application relates to, and claims the priority benefitfrom, U.S. Provisional Patent Application Ser. No. 60/874,254 filed onDec. 12, 2006, filed in English, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

This invention relates to photodetector arrays that use light sensitiveprotein thin films and flexible printed electronic circuits for creatingflexible large-area and wide field-of-view (FOV) imaging systems thatmay be shaped into nonplanar configurations.

BACKGROUND OF THE INVENTION

The functionality and performance of conventional digital cameras areoften limited by the photo-electronic sensor and optical technologiesused to capture two-dimensional planar images of 3D scene. The flatsensor array is composed of numerous single-sized, rectangular shaped,and uniformly distributed light sensing elements fabricated on a rigidsilicon substrate using very-large scaled integration (VLSI) designprocesses. Complex and heavy optical assemblies comprised of specializedlens and mirrors are often required to enable images with a widefiled-of-view (FOV) to be captured. An illustration of a specialty lensis a wide-angle “fish-eye” lens (U.S. Pat. No. 6,304,285B1 and U.S. Pat.No. 4,647,161). However, optical solutions for enlarging the FOV oftenlead to significant image distortion which must be corrected bysophisticated computational algorithms.

The limited FOV provided by commercially available digital sensors hasbeen increased by implementing an active pan-tilt platform thatdynamically repositions the sensor about the center of projection (U.S.Pat. No. 5,627,616). The speed in which the camera can be reorientedduring data acquisition has greatly reduced the effectiveness thesesystems. An alternative approach to creating a wide FOV involvesnumerous cameras pointing in different directions (U.S. Pat. No.5,920,337). However, it is difficult to seamlessly register andintegrate multiple views because the constituent images are produced byunique cameras with different centers of projection. This technology isdifficult to calibrate, prone to alignment errors and requiressignificant hardware for a full 360° view.

The physical restrictions and limited performance capability ofsilicon-based technologies have prompted researchers to look atbiological materials as a means of improving detector characteristics,and simplifying the design and fabrication of non-traditional imagingsystems. Biomaterials such as; bacteriorhodopsin (bR) exhibitphotoresponse characteristics similar to the rhodopsin found inmammalian vision systems including high sensitivity and large dynamicrange. Bacteriorhodopsin is the light-harvesting protein present in theplasma membrane of Halobacterium salinarium. Under anaerobic conditionsthe bacterium's membrane grows purple membrane (PM) patches in the formof a hexagonal two-dimensional crystalline lattice of uniformly orientedbR trimers. It is the crystalline structure that provides bR withchemical and thermal stability thereby making it a useful material fordeveloping artificial vision systems.

A number of different photosensitive devices have been created byimmobilizing PM patches onto a rigid substrate such as silica glasscoated by conductive tin or indium tin oxides, and metal electrodes suchas gold and platinum (J.P. Pat. No. 04006420A2 and U.S. Pat. No.5,260,559). In this context, a 16×16 pixel array of bR photocells wasfabricated on the conductive solid substrate for image detection (U.S.Pat. No. 5,260,559 and U.S. Pat. No. 6,977,160). The differentialresponse exhibited by each bR photocell in the spatial array providedthe necessary photoelectric signal for motion detection. Similarly, aposition-sensitive motion sensor using the image-recording capability ofbR at high pH was patented (J.P. Pat. No. 06235606A2). Recently, thegate terminal of a GaAs-based MOSFET and nano-black lipid membranes (J.Xu et al., 2004, C. Horn and C. Steinem, 2005) has been explored as aviable; technology for micro and nano applications.

A color sensor that consisted of a blue-, green-, and red-sensitivechromoprotein thin films was developed (J.P. Pat. No. 03237769A2).Furthermore, a dynamic adaptive camouflage system was created bymounting the bR photodetectors and display devices on an apparatus anddisplaying a spatially shifted image of the incident ambient light (U.S.Pat. No. 5,438,192).

It would be very advantageous to provide single and multi-spectralbioelectronic sensing technology fabricated on bendable or flexibleplastic substrates. The direct deposit of photosensitive materials ontolow-cost flexible printed electronic circuits would enable designengineers to create innovative lightweight, durable and non-planar imagesensing systems for example, such as spherical or omni-directionalphotodetector array based on bR films can be used in a variety ofimaging applications including motion detection, robotic and vehiclenavigation. It would be very beneficial to develop photo-responsivesensors that can be adhered permanently to non-planar customizedsurfaces.

SUMMARY OF THE INVENTION

The present invention provides a method of designing and manufacturingnon-planar photodetector arrays that exploit light sensitive proteinthin films and flexible printed electronic circuits for creatinglarge-area and wide field-of-view (FOV) imaging systems.

In one aspect of the present invention there is provided a flexiblephotodetector array, comprising of:

a first flexible polymer sheet coated with a continuous electricallyconducting film forming one of an anode and a cathode electrode, asecond flexible polymer sheet having a patterned coating of aelectrically conducting film forming the other electrode, the patternedcoating corresponding to a desired pattern of pixels, at least one ofthe first flexible polymer sheet coated with a continuous, electricallyconducting film and the second flexible polymer sheet having a patternedcoating of a electrically conducting film being substantiallytransparent;

a thin film containing one or more layers of oriented purple membranepatches containing bacteriorhodopsin (bR), or an analog thereof,sandwiched between the first and second flexible polymer sheets with theelectrically conducting film physically contacting one side of the thinfilm and the patterned coating of an electrically conducting filmcontacting an opposed side of the thin film to form a pixel array ofsensing elements; and

the continuous, electrically conducting film and the patterned coatingof an electrically conducting film being electrically connected to asignal processing circuit.

In another aspect of the invention there is provided a method offabricating a flexible photodetector array, comprising the steps of:

a) coating a first flexible polymer sheet with a continuous electricallyconducting film;b) coating a second flexible polymer sheet with a patterned coating ofan electrically conducting film, the patterned coating corresponding toa desired pattern of sensing elements, at least one of the firstflexible polymer sheet coated with a continuous, electrically conductingfilm and the second flexible polymer sheet having a patterned coating ofa electrically conducting film being substantially transparent;c) orienting the purple membrane patches containing (bR), or an analogthereof, and forming a film containing one or more layers of orientedpurple patches and immobilizing the film onto the flexible polymer sheetcoated with the patterned electrically conducting film; andd) affixing the film containing one or more layers of oriented purplemembrane patches between the first and second flexible polymer sheets,with the electrically conducting film physically contacting one side ofthe film and the patterned coating of an electrically conducting filmcontacting an opposed side of the film to form an array of sensingelements and sealing peripheral edges of the first and second flexiblepolymer sheets.

The flexible photoconductor array may be formed into a desired planarshape and then affixed to a non-planar surface. Alternatively it may beshaped into a desired non planar shape without being affixed to asurface.

Thus, to fabricate this novel technology, purple membrane patchesobtained from Halobacterium salinarium are deposited onto a polymerfilm, such as, but not limited to, polyethylene terephthalate (PET)substrate coated with a patterned electrical conductor such as, but notlimited to, indium-tin-oxide (ITO) layer using a deposition technique,such as, but not limited to, the Electrophoretic Sedimentation (EPS)technique in which the step of orienting the purple membrane patchescontaining bR includes locating the purple patches between a flexiblepolymer sheet having the pixel pattern of the conductive film designatedan anode and a continuous planar cathode electrode and applying anelectric field of suitable strength between the planar cathode and anodeelectrodes such that a cytoplasmic side of the purple membrane patchesattaches to the anode electrode electrophoretically and an extracellularside of the purple membrane patches faces to the cathode electrode.

The conductive layer is preferably a metal oxide such as indium tinoxide (ITO) and may be deposited onto the flexible plastic substrate viapulsed laser deposition.

To detect multi-spectral or color images the peak response and signalbandwidth of the individual elements can be modified through chemical orgenetic manipulation. Chemical manipulation may be used to enable thepurple membrane to selectively respond to ultraviolet, visible andinfrared wavelengths. The fabrication process may involve cutting excesssubstrate material located around the sensing elements on the planar PETsheet using for example a computer numerically controlled lasermicromachining in order to increase device flexibility and permit alarger bending radius of the final detector array. Once precisely cutthe flexible sensing array may then be affixed to any surface geometry(e.g. cylindrical, spherical, hemispherical, and freeform) using a nonconductive adhesive.

The following drawings and detailed description provide additionalinformation about the design and fabrication of the invention, itsperformance capabilities, and the technology's advantage for developinginnovative imaging systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The non-planar imaging systems constructed in accordance with thepresent invention will now be described with reference to the drawings,in which:

FIG. 1 a shows a schematic diagram of non-planar imaging arrayformation, looking down from the top, in accordance with the presentinvention;

FIG. 1 b shows a view of the structure of FIG. 1 a along the line A-A;

FIGS. 2 a, 2 b, 2 c and 2 d show the fabrication process for producingthe photoactive array of FIGS. 1 a and 1 b;

FIG. 3 shows absorption spectrum of a bR-ITO-PET film that is normalizedto the baseline absorption of the ITO-PET substrate;

FIG. 4 shows a schematic diagram of a preamplifier configuration for onebR sensing element in which the three dashed boxes include thesimplified bR model, IVC 102 switched integrator and sample/holdcircuitry;

FIG. 5 shows the differential photoresponse (the bottom plot) generatedby a bR sensing element to a step light signal (the top square wavesignal);

FIG. 6 shows measured photovoltage of a bR sensing element over varyingwavelength and illumination power;

FIG. 7 a shows a uniform circle pattern design of ITO electrodes onplanar PET film. Note that the excess substrate material located amongthe sensing element petals on the planar PET sheet is cut by laser inorder to permit a larger bending radius of the sensing array;

FIG. 7 b shows the flexible sensing array is affixed to a hemisphericalgeometry; and

FIG. 7 c and FIG. 7 d show a uniform and a non-uniform hexagonal patterndesigns of ITO conductive layers on planar PET film.

DETAILED DESCRIPTION OF THE INVENTION

The systems described herein are directed, in general, to bioelectronicphotodetector arrays fabricated on flexible electrode substrates.Although embodiments of the present invention are disclosed herein, thedisclosed embodiments are merely exemplary and it should be understoodthat the invention relates to many alternative forms. Furthermore, theFigures are not drawn to scale and some features may be exaggerated orminimized to show details of particular features while related elementsmay have been eliminated to prevent obscuring novel aspects. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting but merely as a basis for the claims and as arepresentative basis for enabling someone skilled in the art to employthe present invention in a variety of manner. For purposes ofinstruction and not limitation, the illustrated embodiments are directedto all embodiments of bioelectronic photodetectors on flexible electrodesubstrates.

As used herein, the term “about”, when used in conjunction with rangesof dimensions, humidities, voltages or other physical properties orcharacteristics is meant to cover slight variations that may exist inthe upper and lower limits of the ranges of dimensions so as to notexclude embodiments where on average most of the dimensions aresatisfied but where statistically dimensions may exist outside thisregion. For example, in embodiments of the present invention bR filmswith thicknesses in a range from 10 to 30 μm are used but statisticallythere may be a few films present outside this range, say at 8 or 9 μm atthe lower end and 31 or 32 μm at the upper end. It is not the intentionto exclude embodiments such as these from the present invention.

As used herein, the term “bioelectronic” means using biomolecules asindependent and functional device elements that are capable ofinterfacing with modern electronic devices as disclosed in S. Bone etal., 1992.

As used herein, the term “bacteriorhodopsin (bR)” means thelight-harvesting protein presented in the plasma membrane ofHalobacterium salinarium.

As used herein, the phrase “or analogs thereof” when referring toanalogs of bacteriorhodopsin (bR) means the chromophore part of bR hasbeen modified through chemical or genetic manipulation.

As used herein, the phrase “signal processing circuit” refers toelectronic circuitry that provide necessary amplification and bufferingfor photoelectric signals generated by a bR sensing element.

As used herein, the term “substantially transparent’ when referring tothe flexible polymer substrates and the electrically conducting layersdeposited on them means high light transmission in the visible spectralrange.

The present invention addresses the problem of producing aphotodetection system which is formed on a flexible substrate system.The present invention describes a non-planar photodetection system thatis comprised of bioelectronic photodetectors and flexible electrodesubstrates. The photodetector is made of a light sensitive proteincomplex formed into in an oriented thin film, which exhibits a change inphotovoltage when exposed to a change in incident light intensity. Thisphotosensitive protein complex is coupled to transparent electrodespatterned onto the flexible polymer film substrate. The electrodestransmit the signal to a preamplifier having high input-impedance. Theoutput signal can be further processed by specialized image processingsystems to explore high-level vision tasks, such as motion detection,edge detection and contrast enhancement.

The photosensitive protein complex may include bacteriorhodopsin (bR) orits analogs. Under illumination, purple membrane patches containing bRgenerate photoelectric signals that can be detected by conventionalelectronics. The use of bR in photoelectric applications requiresimmobilization techniques that can appropriately orient the proteinmolecules and prevent them from denaturing.

Several well-known immobilization methods are available to effectivelyorientate purple membrane patches into a thin film form and can beapplied to the present invention. Methods include deposition of purplemembrane patches by EPS or Langmuir-Blodgett deposition, orencapsulation of oriented bR molecules within polymer gels as disclosedin U.S. Pat. No. 6,284,418 which is incorporated herein by reference inits entirety. An electric field is not necessarily required forLangmuir-blodgett (LB) deposition technique. The LB technique utilizesamphiphilic properties to orient purple membrane (PM) patches as thecytoplasmic side of a PM patch is more hydrophilic than theextracellular side. To improve PM patches' orientation in LB films, anelectric field can be applied across the air-water interface.

A conductive polymer can also be coupled to the thin film of proteincomplex through either chemical or physical links, enhancing electricalconductivity between the photosensitive protein complex and electrodes.

The purple membrane patches containing bR are oriented after laying thepatches down on the array of patterned conductors on one of the flexiblesheets by applying an electric field of suitable strength between theparallel electrodes which are preferably separated by a 1.0 mm plasticspacer. Since the cytoplasmic side of purple membrane patches exhibitsmore negative charges than the extracellular side at pH values above 5,the cytoplasmic side of the membrane patches deposits to the anodeelectrophoretically. The continuous cathode electrode is then lifted andthe bulk water is carefully removed with a pipette. The film containingoriented purple membrane patches is allowed to dry by placing it in ahumidity-regulated chamber for 12 hours. When dry, a PET film with acontinuous ITO coating is then aligned with the dried bR film betweenthe two flexible polymer sheets with the continuous and patternedconductive layer and sealed carefully along the edge by using fastcuring epoxy resin. The continuous cathode electrode may be the flexiblepolymer sheet with the continuous conductive layer or it may be anotherplanar electrode.

Either side of PM patches can face one of two flexible electrodes, onewith a continuous ITO coating and one with a patterned ITO coating. Theelectrodes having a continuous ITO coating that are used in orientationand in the final assembly are not necessary the same, since twodifferent processes are involved.

The protein-based image array of the present invention may be modifiedto detect light over a wide spectral range by shifting spectral responsepeaks of individual sensing elements. Wide-type bR responds most toyellow light. Spectral characteristics can be changed through chemicalor genetic manipulation as disclosed for examples in R. S. H. Liu etal., 1993; and B. Yan et al., 1995. Groups of modified bR sensingelements can be arranged in a predetermined spatial pattern, allowingdetection of both color (multi-spectral) and spatial data. Each elementconfiguration responds to a different spectral range of incident ambientlight. In addition, the spectral ranges overlap, allowing intermediatewavelengths to be identified. The outputs of each sensing element can becoupled to an image processing system. The protein-based image array ofthe present invention is fabricated on flexible substrates coated withconductive materials. Flexible plastic sheets, such as polyethyleneterephthalate (PET), are chosen to replace solid substrate materials.Other flexible polymer materials that could be used for the top andbottom flexible substrates include polyethylene naphthalate (PEN),polycarbonate (PC) and polyimide to mention just a few. It is notnecessary that both polymer films be transparent. However, usingtransparent polymer films on both sides will give more flexibility (ormore choices) when affixing the imaging array to different geometries.

Photodetection devices require a transparent conductive layer with highelectrical conductivity and low visible light absorption. This layer canbe prepared from a wide variety of materials, including dopedsemiconductor oxides of tin, indium, zinc, or cadmium and titaniumnitride ceramics, as well as thin metal layers of silver or gold. Amongthem, the most widely adopted solutions are conducting oxides of tin(SnO₂:F), zinc (ZnO₂:F) and indium (In₂O₃:Sn, ITO). Although indium is acostly raw material and its tin-doped oxide is moderately more difficultto etch than zinc oxide. However, it allows for very low depositiontemperatures (≦200° C.) and provides high conductivity, making it idealfor deposition on thin flexible plastic substrates. As with theelectrodes discussed above, it is not necessary that both electrodes besubstantially transparent. However, using transparent electrodes on bothsides will give more flexibility (or more choices) when affixing theimaging array to different geometries.

The thin film containing purple membrane patches preferably has athickness in a range from about 10 μm to about 30 μm. The first andsecond flexible polymer sheets preferably have a thickness in a rangefrom about 10 μm about 250 μm. The patterned coating of a substantiallytransparent electrically conducting film preferably has a thickness in arange from about 100 Å to about 2000 Å.

The shape and size of the patterned array, as well as the shape and sizeof the individual sensing elements, are key factors to be considered inthe design and manufacture of the flexible photodetector device. Theindividual element shape (e.g. circular, square or hexahedral) mayaffect mechanical properties of the flexible device and thereforecertain shapes may be preferred for a particular end-use application orcut-out pattern. Similarly, the sensing element size may affect themanner in which the PM patches can be located on the substrate. Thedensity of sensing elements directly affects the resolution of theimaging array. In this context, a non-uniform distribution of varioussized sensing elements are also possible to obtain high resolution(small size, high density) in the center and low resolution (large size,low density) in the peripheral region of the image array. Furthermore,the subunits of a sensing element may be formed from two or more of thePM patches with different spectral characteristics.

Preferred methods for coating the flexible substrate include pulsedlaser deposition and magnetron sputtering. Pulsed laser deposition isable to pattern the electrode directly and eliminate the etchingprocess. However, this method is costly and often limited to smallareas. An alternative is to use magnetron sputtering for large-scaleapplications and high volume production. Since photomasks are used forpatterning ITO coating on the plastic film, innovative ITO electrodepattern designs can be developed using a 2D CAD tool. In this manner, itis straightforward to modify the sensing element density, dimensions andgeometry prior to fabrication.

Excess substrate material on the initial flat patterned image array canbe removed using laser cutting technology for increased flexibility.Once cut the flexible sensing array can be affixed to the desiredsurface geometry using a transparent non-conductive adhesive. Light canbe focused onto the non-planar imaging array elements using aconventional convex optical lens or multiple micro lenses. Microlensarrays that are fabricated onto a thin silicon elastomer by using UVexposure can be employed to developing compact-lens sensor that mimicsapposition compound eyes (G. Schmidt and D. T. Moore, 2006).

The signal processing circuitry is designed in the present invention todetect the small photovoltage signal generated by a bR sensing elementthat exhibits significantly high impedance. Two examples include usingtransimpedance amplifiers and switched integrators as the preamplifiersto increase the measured output signal from the individual sensingelements. Each element may be read in parallel from the array or scannedserially, provided that high-impedance transistors are used asamplification components and these devices are appropriately integratedwith each element on the array; Protein-based imaging arrays of thepresent invention can be used as a conformal sheet applied to curvedstructures (e.g. sphere, hemispherical cavity) or flexible materials.

The present invention will now be illustrated with the followingnon-limiting examples.

EXAMPLE

A simple implementation of the protein-based flexible imaging array isshown in FIGS. 1 a and 1 b. The imaging array is comprised of orientedbR films and two PET films, one with a patterned ITO layer coatingcorresponding to a desired pattern of pixels and one with a continuousITO layer. Purple membrane patches containing bR are extracted fromwild-type Halobacterium salinarium by using a standard procedure asdisclosed in D. Oesterhelt and W. Stoeckenius, 1974. In order to removesucrose and salt ions, the purple membrane suspension is purified bydiluting with bi-distilled water and then isolated by centrifugation.The ultracentrifuge operates at 22400 rpm at 4° C. for 30 minutes with a50-Ti rotor. After the rinsing cycle, the supernatant is carefullyremoved and the PM pellet is re-suspended in bi-distilled water. Thiscentrifugation process is repeated two more times and the final purplemembrane pellet is suspended in bi-distilled water to achieve a desiredconcentration of 16 mg/ml. This sample is then softly sonicated for 10seconds to break up particle aggregation. Upon the completion of threerinse cycles, the pH of the suspension is roughly 6.7, which eliminatesthe need for acidification or alkalization of the sample.

The flexible substrate is made of a flexible 175 μm thick PET film whichprovides high light transmission (typically >86%) over the visiblespectral range. A patterned ITO layer is deposited onto the flexiblesubstrate via pulsed laser deposition. The ITO coated film has a surfaceresistance of 35 ohms/sq. In an embodiment, the ITO electrode isimprinted as a 4×4 pixel array pattern where each pixel is 2 mm×2 mm andis separated by 1 mm between neighboring pixels. Independent ITO wires,each 300 μm wide, connect the pixel with a connection terminal alongsubstrate edge. The overall area of a patterned ITO electrode is 15 mmby 23 mm. It will be appreciated that in the patterned coating of thesubstantially transparent and electrically conducting film (e.g. ITO),the patterned coating corresponds to any desired pattern or array, ofpixels, which may also be referred to as a pixilated array.

In the construction of FIGS. 2 a and 2 b, the patterned electrodesurface is first cleaned of all residues, and then 100 μL of PMsuspension is spread evenly over the patterned surface. A 1.0 mm plasticspacer is placed around the array to separate the two electrodes, wherethe bottom ITO electrode is patterned and the top ITO electrode iscontinuous, are connected to the positive and negative terminals of thepower supply as shown in FIG. 2 c. An electric field of 40 V/cm is thenapplied. The top electrode is lifted and the remaining bulk water iscarefully removed with a pipette. After 12 hours of drying, the topelectrode is attached and sealed carefully along the edge by usingfast-curing epoxy resin as shown in FIG. 2 d.

The fabrication conditions such as the electric field intensity,exposure time and ambient drying humidity all affect the photoelectricresponse and topology of the bR film. Experimental observations haveshown that noticeable aggregation of bR molecules can occur at very lowelectric field intensities, forming poorly oriented films and diminishthe photoelectric response. Excessively high electric field intensitiesreduce aggregation, but also reduce sensor performance due to proteindegradation. An intermediate electric field in a range from about 20V/cm to about 60 V/cm are useful, preferably applying a field ofapproximately 40 V/cm for about five minutes produces the best results.Relative humidity in the range of 50%˜60% typically yields the bestphotoelectric signals. Constant humidity can be achieved by drying thefilm in the presence of vapor generated by a saturated salt solution.

Fabrication quality is verified by measuring the absorption spectrum ofthe bR film deposited on the ITO-PET substrate. An ultraviolet-visiblespectrophotometer (Varian, Cary 50Bio) is used to record the absorptionspectra. FIG. 3 shows the spectrum of bR-ITO-PET film that is normalizedto the baseline absorption of ITO-PET substrate. A broad peak, from 550nm to 580 nm, is observed for the bR-ITO-PET film. Successful depositionshows that the 570 nm absorption peak found in the purple membranesuspension is also preserved in the bR-ITO-PET film.

An equivalent circuit model of a bR sensing element is given in the leftdashed box shown in FIG. 4. The components in this model correspond toanalogues in the bR element's physical model. The overall behavior ofthe bR molecule is that of a current generator, I(t). Considering thefinite resistance of protein medium, R_(s), the current source can alsobe described by an equivalent electric potential source, E_(ph)(t).Since the lipids within the membrane exhibit a high dielectric constant,they can be modeled as the combination of a series capacitor C_(l) and aseries resistor R_(l). The parallel combination of resistor R_(m) andcapacitor C_(m) with the current source describes the behavior innon-illuminated bR molecules in conjunction with lipids that areperpendicular to the bR orientation direction.

The resistance of a bR element is in the range of 10¹⁰-10¹²Ω, thereforeits performance highly depends on the electronic circuitry used tocondition and amplify the extremely-small signal. A highly optimizedelectronic circuit with high input-impedance and low noise amplificationis designed. The proposed design employs a switched integrator as apreamplifier for each sensing element. FIG. 4 shows a detailed circuitdiagram in which a precision switched integrator, IVC102, combines a FETop amp, integrating capacitors and low leakage FET switches onto asingle chip. The sensing element output connects to the inverting inputvia a sample switch S₁, where the non-inverting input is grounded. Thereset switch S₂ is in parallel with the integrating capacitors toprovide a discharge path. Amplifier output is connected to a holdingcapacitor via a readout switch S₃. The digital timing inputs to S₁-S₃are compatible with standard CMOS or TTL logic signals. A PIC12F675microcontroller is used to control the digital timing functions. Amulti-channel readout architecture is used to transfer the informationprocessed by each element to the processing circuitry. This designsimplifies array fabrication since no shift registers need to beconsidered at this time. The multi-channel design also reduces theinformation to be transferred and increases the transfer bandwidth.

FIG. 5 shows a differential photoresponse generated by a bR sensingelement when illuminated by a step light signal. It illustrates thatpolarity of the photoresponse changes at the rising and falling edges ofthe input step signal, which corresponds to the charge displacementinside the bR film. The peak of the differential response variesdirectly with changes in light intensity provided that saturation doesnot occur. The magnitude of such response is also dependent upon thewavelength of the incident light. Wavelength dependent response ismeasured using a tunable Argon/Krypton laser system that produces sevenwavelengths covering the whole visible range. FIG. 6 shows experimentaldata plotted in a 3D space, where the measured photovoltage is afunction of both illumination power and wavelength.

Accordingly, the present invention provides a novel design for aphotodetection device which functions as a result of the photosensitiveproperties of proteins and which allows for detection a wide range ofincident light intensities and wavelengths in a wide FOV. The inventionwill now be described using the following non-limiting examples ofproducing hemispherical imaging system which is meant to be illustrativeonly and not limiting to the scope of the present invention.

While the present invention is described with respect to particularexamples, it is understood that the present invention is not limited toany one of the embodiments shown herein. The invention as claimedincludes variations from the particular examples and preferredembodiments described herein, as will be apparent to one of skill in theart.

The protein-based design and fabrication methodology would enable avariety of geometries of photodetection systems to be created. Ahemispherical imaging system that can emulate the characteristics ofeither single aperture eye or apposition compound eye is given as anexample.

The ITO electrode pattern is designed as shown in FIG. 7 a. This designis transferred to an image, which is then used to develop a photomask. Aconductive ITO layer is deposited onto a planar PET substrate via pulsedlaser deposition. Excess substrate material located along the sensingelement petals on the planar PET sheet is cut by laser to permit alarger bending radius of the sensing array. Thin layers of purplemembrane patches containing bR is then immobilized onto the patternedITO electrode using one of the preferred methods. The individualelements can be made of wild-type bR with its analogs thereby achievinga wide absorption range. Purple membrane patches can also beencapsulated into organic/inorganic material matrix to achieved enhancedrobustness. A continuous ITO-PET film with the same shape is carefullyaligned on the top of bR-ITO-PET film and sealed along the edges using atransparent adhesive. As shown in FIG. 7 b, the flexiblePET-ITO-bR-ITO-PET imaging array is affixed to a hemispherical geometry.

Non uniform distribution of light sensing elements is quite common inbiological vision systems. The photoreceptors on the hemisphericalsurface of a single aperture eye found in higher order animals reflect acomplex spatial pattern that permits a dense concentration offrequency-selective receptors along the optic axis and a widerdistribution of intensity sensitive receptors in the peripheral regions.The present invention provides a promising approach to mimic suchaspects of biological vision systems. FIG. 7 c and FIG. 7 d show uniformand non-uniform hexagonal pattern designs of ITO conductive layers on aplanar PET film.

Single and multi-spectral bioelectronic sensing technology fabricated onbendable plastic substrates as disclosed herein offers a number ofsignificant advantages over conventional silicon-based microelectronics(e.g. CMOS) and bR-based devices manufactured on rigid substrates, whichinclude reduction in spatial requirements, weight, electrical powerconsumption, thermal heat loss, system complexity, and fabrication cost.

The direct deposit of photosensitive materials onto low-cost flexibleprinted electronic circuits also enables design engineers to createinnovative lightweight, durable and non-planar image sensing systems.For example, spherical or omni-directional photodetector array based onbR films can be used in a variety of imaging applications includingmotion detection, robotic and vehicle navigation, surveillance of workenvironments, and biosensing. The non-planar shape of the sensing arrayprovides a number of unique advantages such as increased FOV for singleaperture camera systems, and direct sensing capability for a compactapposition “compound eye”.

By manufacturing patterned imaging arrays on bendable substrates it isalso possible to develop photo-responsive sensors that can be adheredpermanently to non-planar customized surfaces or, in the near future,rolled up when not in operation. An example is that a large areaflexible bioelectronic sensor array can be used to create an“intelligent skin” which can be adhered directly to the shell of land,air and marine vehicles.

As used herein, the terms “comprises”, “comprising”, “including” and“includes” are to be construed as being inclusive and open ended, andnot exclusive. Specifically, when used in this specification includingclaims, the terms “comprises”, “comprising”, “including” and “includes”and variations thereof mean the specified features, steps or componentsare included. These terms are not to be interpreted to exclude thepresence of other features, steps or components.

The foregoing description of the preferred embodiments of the inventionhas been presented to illustrate the principles of the invention and notto limit the invention to the particular embodiment illustrated. It isintended that the scope of the invention be defined by all of theembodiments encompassed within the following claims and theirequivalents.

REFERENCES CITED

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OTHER PUBLICATIONS

-   J. Xu, P. Bhattacharya, G. Váró, “Monolithically integrated    bacteriorhodopsin/semiconductor opto-electronic integrated circuit    for a bio-photoreceiver”, Biosensors & Bioelectronics, vol. 19, pp.    885-892, 2004.-   C. Horn, C. Steinem, “Photocurrents generated by bacteriorhodopsin    adsorbed on nano-black lipid membranes”, Biophys. J. vol. 89, pp.    1046-1054, 2005.-   S. Bone and B. Zaba, “Bioelectronics”, Chichester: John Wiley & Sons    Ltd., 1992.-   R. S. H. Liu, E. Krogh, X.-Y. Li, D. Mead, L. U. Colmenares, J. R.    Thiel, J. Ellis, D. Wong and A. E. Asato, “Analyzing the red-shift    characteristics of azulenic, naphthyl, other ring-fused and retinyl    pigment analogs of bacteriorhodopsin”, Photochem. Photobiol., Vol.    58, pp. 701-705, 1993.-   B. Yan, J. L. Spudich, P. Mazur, S. Vunnam, F. Derguini and K.    Nakanishi, “Spectral tuning in bacteriorhodopsin in the absence of    counterion and coplanarization effects”, J Biol Chem, vol. 270, pp.    29668-29670, 1995.-   G. Schmidt and D. T. Moore, “Formable compound micro-lens arrays”,    SPIE Newsroom, 2006. http://newsroom.spie.org/x4676.xml (DOI:    10.1117/2.1200609.0405)-   D. Oesterhelt and W. Stoeckenius, “Isolation of the cell membrane of    alobacterium halobium and its fractionation into red and purple    membrane”, Methods. Enzymol., vol. 31, pp. 667-678, 1974.

1. Flexible photodetector array, comprising of: a first flexible polymersheet coated with a continuous electrically conducting film forming oneof an anode and a cathode electrode, a second flexible polymer sheethaving a patterned coating of a electrically conducting film forming theother electrode, the patterned coating corresponding to a desiredpattern of pixels, at least one of the first flexible polymer sheetcoated with a continuous, electrically conducting film and the secondflexible polymer sheet having a patterned coating of a electricallyconducting film being substantially transparent; a thin film containingone or more layers of oriented purple membrane patches containingbacteriorhodopsin (bR), or an analog thereof, sandwiched between thefirst and second flexible polymer sheets with the electricallyconducting film physically contacting one side of the thin film and thepatterned coating of an electrically conducting film contacting anopposed side of the thin film to form a pixel array of sensing elements;and the continuous, electrically conducting film and the patternedcoating of an electrically conducting film being electrically connectedto a signal processing circuit.
 2. The flexible photodetector arrayaccording to claim 1 wherein the oriented purple membrane patchescontaining bR are oriented such that cytoplasmic sides of all purplemembrane patches face to one direction and extracellular sides of allpurple membrane patches face to the opposite direction, and whereineither the cytoplasmic sides or the extracellular sides beingimmobilized to the flexible polymer sheet having a patterned coating ofan electrically conducting film.
 3. The flexible photodetector arrayaccording to claim 1 wherein the purple membrane patches containing bRare extracted from wild-type Halobacterium salinarium.
 4. The flexiblephotodetector array according to claim 1 wherein the purple membranepatches containing bR have a thickness in a range from about 10 μm toabout 30 μm.
 5. The flexible photodetector array according to claim 1wherein the first and second flexible polymer sheets have a thickness ina range from about 10 μm about 250 μm.
 6. The flexible photodetectorarray according to claim 1 wherein the patterned coating of anelectrically conducting film have a thickness in a range from about 100Å to about 2000 Å.
 7. The flexible photodetector array according toclaim 1 wherein said first and second flexible polymer sheets are madefrom a polymer selected from the group consisting of polyethyleneterephthalate (PET), polyethylene naphthalate (PEN), polycarbonate (PC)and polyimide.
 8. The flexible photodetector array according to claim 1wherein a group of bR sensing elements with predetermined shapes andsizes forms one single photodetector, and a subunit of the photodetectorwherein the bR sensing element is formed from two or more of said purplemembrane patches.
 9. The flexible photodetector array according to claim1 wherein said group of modified bR sensing elements are arranged inaccordance with at least one characteristic of the purple membranepatches.
 10. The flexible photodetector array according to claim 9wherein said at least one characteristic is a spectral characteristic.11. The flexible photodetector array according to claim 1 wherein saidfirst and second flexible polymer sheets enclosing said bR thin filmshas a pre-selected planar shape and is affixed to a non-planar surface.12. The flexible photodetector array according to claim 11 wherein saidnon planar surface is any one of cylindrical, spherical, hemispherical,or freeform geometries.
 13. The flexible photodetector array accordingto claim 11 wherein said non planar surface is a convex or concavesurface.
 14. The flexible photodetector array according to claim 1wherein said thin film containing one or more than one layers oforiented purple membrane patches forming said pixel array comprise twoor more types of purple membrane patches with different bR characterizedby different wavelength sensitivities spanning the spectral region frominfrared to ultraviolet wavelengths, said two or more types of purplemembrane patches being arrayed in a desired pattern on said patternedcoating of an electrically conducting film.
 15. The flexiblephotodetector array according to claim 1 wherein said electricallyconducting film and the patterned coating of a electrically conductingfilm are made of a conducting metal oxide.
 16. The flexiblephotodetector array according to claim 15 wherein said conducting metaloxide is selected from the group consisting of tin oxides (SnO₂:F), zincoxides (ZnO₂:F) and indium oxides In₂O₃:Sn (ITO).
 17. The flexiblephotodetector array according to claim 1 including a plurality ofoptical components affixed to at least one of said first and secondflexible polymer sheets with each of said optical components beingassociated with a particular bR sensing element or a group of bR sensingelements, each optical component being shaped and configured forfocusing incident light onto its associated bR sensing element or agroup of bR sensing elements.
 18. The flexible photodetector arrayaccording to claim 17 wherein said optical component is a microlens. 19.The flexible photodetector array according to claim 1 including anamplifier electrically connected to each sensing element to amplifyelectric signals generated by each sensing element.
 20. The flexiblephotodetector array according to claim 19 wherein said amplifier is atransimpedance amplifier.
 21. The flexible photodetector array accordingto claim 19 wherein said amplifier is a switched integrator.
 22. Theflexible photodetector array according to claim 1 including parallelreadout circuitry connected to each sensing element of said array. 23.The flexible photodetector array according to claim 1 including scanningreadout circuitry connected to said array of pixels.
 24. A method offabricating a flexible photodetector array, comprising the steps of: a)coating a first flexible polymer sheet with a continuous electricallyconducting film; b) coating a second flexible polymer sheet with apatterned coating of an electrically conducting film, the patternedcoating corresponding to a desired pattern of sensing elements, at leastone of the first flexible polymer sheet coated with a continuous,electrically conducting film and the second flexible polymer sheethaving a patterned coating of a electrically conducting film beingsubstantially transparent; c) orienting the purple membrane patchescontaining (bR), or an analog thereof, and forming a film containing oneor more layers of oriented purple patches and immobilizing the film ontothe flexible polymer sheet coated with the patterned electricallyconducting film; and d) affixing the film containing one or more layersof oriented purple membrane patches between the first and secondflexible polymer sheets, with the electrically conducting filmphysically contacting one side of the film and the patterned coating ofan electrically conducting film contacting an opposed side of the filmto form an array of sensing elements and sealing peripheral edges of thefirst and second flexible polymer sheets.
 25. The method according toclaim 24 wherein the purple membrane patches containing bR are orientedusing any one of Electrophoretic Sedimentation (EPS), Langmuir-Blodgettdeposition and encapsulation into organic/inorganic material matrix. 26.The method according to claim 24 wherein said step of orienting thepurple membrane patches containing bR includes locating the purplepatches between the second flexible polymer sheet designated an anodeand a continuous planar cathode electrode and applying an electric fieldof suitable strength between the planar cathode and anode electrodessuch that a cytoplasmic side of the purple membrane patches face to theanode electrode electrophoretically and an extracellular side of thepurple membrane patches face to the cathode electrode.
 27. The methodaccording to claim 26 wherein the purple membrane patches containing bRare oriented by applying an electric field in a range from about 20 V/cmto about 60 V/cm.
 28. The method according to claim 27 wherein theelectric field is about 40 V/cm and is applied for about five minutes.29. The method according to claim 24 wherein after step c) and prior tostep d) the film containing oriented purple membrane patches is dried ina humidity-regulated chamber for about 12 hours.
 30. The methodaccording to claim 29 wherein the film containing oriented purplemembrane patches is dried for about 12 hours.
 31. The method accordingto claim 24 wherein the purple membrane patches containing bR areextracted from wild-type Halobacterium salinarium.
 32. The methodaccording to claim 24 including chemically or genetically modifyingindividual purple membrane patches containing bR for modifying spectralcharacteristics of the individual purple membrane patches over theultraviolet, visible and infrared wavelengths regions, and once theyhave been oriented, laying down said chemically or genetically modifiedindividual purple membrane patches in a preselected pattern of sensingelements.
 33. The method according to claim 24 wherein a group of bRsensing elements with predetermined shapes and sizes forms one singlephotodetector, and a subunit of a photodetector wherein the bR sensingelement is formed from two or more of said purple membrane patches. 34.The method according to claim 33 wherein said group of modified bRsensing elements are arranged in said array in accordance with at leastone characteristic of the purple membrane patches.
 35. The methodaccording to claim 34 wherein said at least one characteristic is aspectral characteristic.
 36. The method according to claim 24 includinga step of forming the flexible photoconductor array into a desiredplanar shape and affixing the shaped flexible photoconductor array to anon-planar surface.
 37. The method according to claim 36 wherein saidnon planar configuration is any one of cylindrical, spherical,hemispherical, or freeform geometries.
 38. The method according to claim36 wherein said non planar non-planar surface is a convex or concavesurface.
 39. The method according to claim 24 wherein said first andsecond flexible polymer sheets are made from a polymer selected from thegroup consisting of polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), polycarbonate (PC) and polyimide.